In general, an extrusion such as extruded polyvinyl chloride (PVC) pipe is formed by feeding plastic into an extruder where it is subjected to high temperatures to create a molten substrate. The substrate then proceeds through a feed tube, at the end of which additional material known as capstock can be applied if desired. The process of adding additional material to the substrate is known as co-extrusion. The substrate whether or not capstock coated is known as melt. The melt proceeds through an extrusion head, at the end of which the melt passes through a die. The die contains the circular cross-sectional profile shape to be extruded. The melt hardens as it exits the die in the desired cross-sectional form. The hardened material forms a tube that can grow to arbitrary length as additional melt is extruded.
Normally, PVC pipe is produced by extruding molten plastic through a single die of an extruder. In some cases, two PVC pipes are produced simultaneously by extruding molten plastic through a Y-block, a pair of extrusion heads and a pair of dies.
The ultimate shape of the extrusion is determined by a melt flow passage in the die between a bushing which surrounds a pin or mandrel. For circular pipe, the pin is circular in cross-section and an opening in the bushing which surrounds the pin is circular. To obtain a non-circular cross-section, such as a square, a circular extrusion is typically reshaped through a transition bushing and pin which are circular at the inlet and square, for example, at the outlet.
One problem that arises, based on the current method and applied die technology is that it is extremely difficult to consistently extrude non-circular cross-section profiles, and correction of tolerance errors inside the extrusion die require interruption of the extrusion process. With circular cross-sections, adjustment may be provided to shift the bushing relative to the pin to obtain an extrusion of uniform thickness. However, with non-circular cross-sections, where rotation of the bushing relative to the pin cannot be tolerated, prior approaches are not feasible.
In accordance with the invention, there is provided a method and an extrusion die for adjusting the wall thickness of a non-circular cross-sectional profile without stopping the extrusion process.
The invention achieves this result by providing an extrusion die which contains a bushing plate, a profile pin and a first adjustment plate. The bushing plate has a flow path which shapes the exterior profile of the melt. The profile pin is located within the flow path of the bushing pin and shapes the interior profile of the melt. The first adjustment plate faces the bushing plate and surrounds the profile pin and may be moved in a direction transverse to the flow of the melt to provide a shift of the non-circular cross-sectional profile of the flowing melt. The movement of the first adjustment plate is restricted to prevent rotation relative to the bushing plate or may restrict movement along a first transverse axis.
A second adjustment place which faces the first adjustment plate and surrounds the profile pin may be moved orthogonal to the first adjustment plate to provide a shift of the non-circular profile of the flowing melt. The movement of the second adjustment plate is restricted to prevent rotation relative to the first adjustment plate.
A first bushing plate defines a flow path which maintains the circular cross-sectional profile exterior of the flowing melt. A second bushing plate defines a flow path which shapes the circular melt exterior to the desired non-circular cross-sectional profile exterior of the flowing melt. A third bushing plate defines a flow path which maintains the desired non-circular cross-sectional profile exterior of the flowing melt.
A first section of the profile pin defines a flow path which maintains the circular cross-sectional interior of the flowing melt. A second section of the profile pin defines a flow path which shapes the circular melt interior to the desired non-circular cross-sectional profile interior of the flowing melt. A third section of the profile pin defines a flow path which maintains the desired non-circular cross-sectional profile interior of the flowing melt.
A bushing plate comprising non-circular cross-sectional surfaces defining a flow path through the bushing plate to maintain the desired non-circular cross-sectional profile. Opposing protrusions at a distal end from a face of the bushing plate defining at a distal end an adjustment channel which receives shoulders of a first adjustment plate and locates the first adjustment plate therein to prevent rotation of the first adjustment plate relative to the bushing plate.
A first adjustment plate comprising non-circular cross-sectional surfaces defining a flow path through the first adjustment plate to maintain the desired non-circular cross-sectional profile. Shoulders at a proximal end from a face of the first adjustment plate which maybe moved within an adjusting channel of the bushing plate to prevent rotation of the first adjustment plate relative to the bushing plate. Opposing adjusting channels from a distal face which receive shoulders of the second adjustment plate and locate the second adjustment plate therein to prevent rotation of the second adjustment plate relative to the first adjustment plate.
A second adjustment plate comprising non-circular cross-sectional surfaces defining a flow path through the second adjustment plate to maintain the desired non-circular cross-sectional profile. Opposing shoulders at a proximal end from a face of the second adjustment plate moveable within adjusting channels of the first adjustment plate to prevent rotation of the second adjustment plate relative to the first adjustment plate.